The number of migrants reaching Italy's southern coastline after a perilous crossing from Africa has soared by more than 50% this year, according to UN figures, highlighting the mounting moral and diplomatic dilemma that will face countries in southern Europe in 2009.

Even before another wave of landings in recent days, the number of arrivals this year soared to 33,000 - 13,000 more than in the whole of 2007, according to figures supplied yesterday by the UN refugee agency, UNHCR. Up to 500 more, many of them asylum seekers from countries such as Somalia, are known or feared to have died before reaching Italian shores.

Since December 24, a further 2,400 people have come ashore in the Pelagic islands between Libya and Sicily. The most recent landings took place on Sunday after a boat carrying 331 migrants crashed into rocks fringing the tiny volcanic island of Linosa.

Mediterranean crossings in the depths of winter were once a rarity. But Laura Boldrini, UNHCR's spokeswoman in Italy, said these had become "increasingly frequent. That is more dangerous because the sea conditions are apt to change more abruptly in the colder months." She said another factor adding to the perils facing migrants was that they were being consigned by traffickers to "ever-more unseaworthy vessels, like homemade semi-dirigibles and ageing fishing boats".

Italy's interior minister, Roberto Maroni, threatened yesterday to fly new arrivals to their country of origin. He said flights would begin "tomorrow or at the latest the day after tomorrow". But it remained unclear how such flights could be put into operation within the terms of international treaties on the rights of asylum seekers. According to UNHCR, the seaborne arrivals habitually include an unusually high proportion of people later granted refugee status.

Maroni also said he would meet counterparts from Cyprus, Greece and Malta on 13 January "to find a common strategy". But his attempts to address the growing problem were not enough for the Italian right, and there were signs last night of a rift in the Silvio Berlusconi government over the issue. Though Maroni claimed the majority of unauthorised immigrants were people who arrive legally in Italy and then overstay their visas, the Mediterranean landings, with their dramatic images of exhausted survivors, have cast doubt on Berlusconi's electoral promise to deal with immigration.

The arrival of so many immigrants in recent days is a considerable embarrassment to his government, especially since many of its voters had believed a much-vaunted deal with Libya would stem the flow. The shadow interior minister, Marco Minniti, meanwhile piled on the pressure. He said: "What is happening at the moment is proof that the government's hardline policy on clandestine immigration has failed miserably."

On 30 August, Berlusconi signed with the Libyan leader, Colonel Muammar Gaddafi, what was hailed as a historic deal to heal the wounds left by Italy's colonial occupation of the north African state. Rome agreed to make substantial long-term investments in Libya's infrastructure and government officials made it clear they expected Gaddafi to implement joint coastal patrols that had been agreed in an earlier accord with the previous, centre-left government.

The latest wave of arrivals had prompted complaints from backbench MPs in the governing majority, who suspect the Libyan leader of holding out for further concessions before cracking down on the trade in human beings from his country's Mediterranean ports.

One described Gaddafi as "the worst sort of blackmailer".

Maroni hit back at a cabinet colleague - the defence minister, Ignazio La Russa - who had suggested that "getting tough with the Libyans does not achieve anything". Maroni, who belongs to the anti-immigrant Northern League, said in a radio interview: "We have applied and will continue to apply pressure."

Slightly more than half of those who came ashore on Linosa on Sunday were flown to Lampedusa, the biggest of the Pelagic islands. A senator from the islands, Angela Maraventano, said those who remained had increased Linosa's population by a third. "We shall put these desperate people up in churches and schools," she told La Repubblica newspaper.